Currently, 3rd generation cellular communication systems are being installed to further enhance the communication services provided to mobile phone users. The most widely adopted 3rd generation communication systems are based on Code Division Multiple Access (CDMA) and Frequency Division Duplex (FDD) or Time Division Duplex (TDD) technology. In CDMA systems, user separation is obtained by allocating different spreading and/or scrambling codes to different users on the same carrier frequency and in the same time intervals. This is in contrast to time division multiple access (TDMA) systems, where user separation is achieved by assigning different time slots to different users. An example of communication systems using these principles is the Universal Mobile Telecommunication System (UMTS™).
In order to provide enhanced communication services, the LTE version of 3rd generation cellular communication systems are designed to support a variety of different and enhanced services. One such enhanced service is multimedia services. The demand for multimedia services that can be received via mobile phones and other handheld devices is set to grow rapidly over the next few years. Multimedia services, due to the nature of the data content that is to be communicated, require a high bandwidth. The typical and most cost-effective approach in the provision of multimedia services is to ‘broadcast’ the multimedia signals, as opposed to sending the multimedia signals in an unicast (i.e. point-to-point) manner. Typically, tens of channels carrying say, news, movies, sports, etc., may be broadcast simultaneously over a communication network. Further description of LTE, can be found in Sesia, Toufik, Baker: ‘LTE—The UMTS™ Long Term Evolution; From Theory to Practice’, page 11. Wiley, 2009.
As radio spectrum is at a premium, spectrally efficient transmission techniques are required in order to provide users with as many broadcast services as possible, thereby providing mobile phone users (subscribers) with the widest choice of services. It is known that broadcast services may be carried over cellular networks, in a similar manner to conventional terrestrial Television/Radio transmissions. Thus, technologies for delivering multimedia broadcast services over cellular systems, such as the evolved Mobile Broadcast and Multicast Service (eMBMS) for the LTE aspect of E-UTRA, have been specified over the past few years. In these broadcast cellular systems, the same broadcast signal is transmitted over non-overlapping physical resources on adjacent cells within a conventional cellular system. Consequently, at the wireless subscriber unit, the receiver must be able to detect the broadcast signal from the cell it is connected to. Notably, this detection needs to be made in a presence of additional, potentially interfering broadcast signals, transmitted on the non-overlapping physical resources of adjacent cells.
To improve spectral efficiency, broadcast solutions have also been developed for cellular systems in which the same broadcast signal is transmitted by multiple cells but using the same (i.e. overlapping) physical resources. In these systems, cells do not cause interference to each other as the transmissions are arranged to be substantially time-coincident, and, hence, capacity is improved for broadcast services. Such systems are sometimes referred to as ‘Single Frequency Networks’, or ‘SFNs’. In SFN systems, a common cell identifier (ID) is used to indicate those (common) cells that are to broadcast the same content at the same time. In the context of the present description, the term ‘common cell identifier’ encompasses any mechanism for specifying SFN operation, which may in some examples encompass a use of, say, a single scrambling code.
The LTE eMBMS feature was introduced to the 3GPP™ standard specifications in Release 9. When enabled, a wireless subscriber unit (referred to as user equipment (UE) in 3GPP™ parlance) is informed as to those subframes that have been allocated to eMBMS transmissions. The MBMS control channel (MCCH) and the MBMS traffic channel (MTCH) are multiplexed together in these subframes. The MCCH can change on a periodic basis, known as the MCCH modification period. In order to provide an efficient mechanism to notify UEs of upcoming changes to the MCCH, an MCCH change notification is transmitted in the modification period prior to the MCCH change. Thus. UEs are able to determine in advance that the MCCH information will have changed from the MCCH modification period boundary. There may be a one-to-one mapping between an MCCH and an MBSFN area; an MBSFN area is a group of cells coordinated to achieve an SFN transmission.
A MCCH change notification is contained in a physical downlink control channel (PDCCH) transmission located in an MBMS subframe. The PDCCH is located in the common search space of the PDCCH transmission space. A downlink control information (DCI) Format 1C of 8 bits is firstly padded, then a cyclic redundancy check (CRC) is added, before it is convolutionally encoded and mapped to the PDCCH. In order to distinguish this DCI from other DCI mapped to the PDCCH common search space, the CRC is scrambled by the unique MBMS radio network temporary identifier (M-RNTI).
Carrier Aggregation (CA) was introduced in Rel. 10 of the 3GPP™ standards. CA supports the aggregation of two or more component carriers (CC), up to a total of five CCs, which advantageously provide wider transmission bandwidths of, say, up to 100 MHz, for some UEs to utilise. CA allows a UE to simultaneously receive one or multiple component carriers, depending on the UE's capabilities. A UE that is capable of the aggregation of multiple component carriers can be configured to be cross-carrier scheduled, e.g. the allocation information for resources on one component carrier is transported on a different component carrier. It is also possible to aggregate a different number of component carriers of possibly different bandwidths in the uplink (UL) and the downlink (DL) channels. In typical TDD deployments, the number of component carriers and the bandwidth of each component carrier in UL and DL will be the same.
However, when a UE is not cross-carrier scheduled, then the UE is configured to read the component carrier (CC) physical downlink control channel (PDCCH) on each serving-cell in order to determine whether or not a resource allocation is present on that component carrier. If a UE is cross-carrier scheduled, a carrier indicator field (CIF) can be semi-statically configured to enable cross-carrier UL and DL assignment of frequencies for use, for example using the PDCCH in a first component carrier (CC1) to allocate a physical downlink shared channel (PDSCH) resource in a second component carrier (CC2).
When operating in an aggregated carrier mode, each UE is configured with one or more serving cells. Among these serving cells, one is designated the primary cell (Pcell) and any others are designated as secondary cells (Scells). The Pcell designation is UE-specific and certain attributes are associated with the Pcell. Each serving-cell is designed to be Release 8/9 backwards compatible.
However, the Release 10 of the 3GPP™ standard specification does not support CIF in the DCI Format 1C, as used in Release 9 eMBMS. Furthermore, eMBMS has not been discussed as part of Release 10 of the 3GPP™ standard. Therefore, no solution to supporting cross-carrier scheduling of eMBMS signalling exists, particularly in a case where a UE desiring eMBMS content is capable of carrier aggregation. In addition, within Release 10 of the 3GPP™ standard, a UE is not expected to decode the common search space on a secondary cell (Scell). Hence, amongst other factors, the LTE specification is silent on providing for MBMS control channel change notification on any carrier other than the primary carrier/cell (Pcell) (as defined for Release 9).
Referring now to FIG. 1, a layer-2 structure of the LTE standard of a downlink configured with carrier aggregation is shown, as disclosed in 3GPP™ specification TS 36.300. As shown, that whilst radio bearers that are mapped to DL shared transport channels can be configured for carrier aggregation (see for example component carriers 1, . . . ,x for UE1), no such mapping can be configured for radio bearers that are mapped to the multicast transport channel (MCH). In Release 10 carrier aggregation, each component carrier is compatible with LTE Release 8/9. Consequently, each component carrier can carry MBMS transmissions. However, there is no mechanism for cross carrier notification or other carrier aggregation features that may be desirable to apply to MBMS.
Consequently, current techniques are suboptimal. In particular, there is no current technique to enable a single service or collection of MBMS logical channels belonging to a single MBSFN area, and hence controlled by a single MCCH, to be mapped to multiple, i.e. at least two, aggregated component carriers.
Further, there is no current technique to provide cross carrier indication of an MCCH change notification for LTE CA, for a situation where a UE is operating on a primary cell (Pcell) and is configured with at least one secondary cell (Scell).